Salinity Profiles in Coastal Aquifers: A Characterization Framework for Field Measurements

Abstract

The freshwater-seawater mixing zone (“interface”) is affected by tides, seawater intrusion, submarine groundwater discharge and aquifer heterogeneities. Our current understanding is based primarily on theoretical studies involving numerical simulation and sand-tank experimentation, with limited field analyses of interface characteristics in coastal aquifers that serve as important freshwater reservoirs. The most common field evidence for interface characteristics is derived from electrical conductivity (EC) profiles, often obtained from long-screen wells. However, the EC profiles from long-screen wells have been assessed previously using only simple statistics like the depth to 50% of seawater concentration. This study introduces a methodology for characterizing EC profiles using curve fitting and objective assessment of multiple interface attributes, with application to 170 EC profiles from the Lower Burdekin Delta, Australia. The adopted fitting function showed an excellent match (mean R2 = 0.99). Fitting parameters were linked to the curvatures of transitions from fresh to brackish water and from brackish water to seawater, the elevation and gradient of the mid-salinity value, and the thickness of the mixing zone. Field EC profiles showed approximately linear relationships between head changes averaged over the previous 5 months and the elevation of 5 mS/cm isochlor for some wells. The slope of this relationship was approximately one, which is approximately 1/40th of the value arising from the Ghyben-Herzberg relation. The framework for characterizing salinity profiles presented in this article offers a new, objective approach for investigating the interface, providing future studies with a technique for examining relationships between interface behavior and key controlling forces.